Abstract Prostate cancer is the second leading cancer in incidence and mortality in American men. Responsive to androgen ablation therapy initially, almost all patients progress to a castration resistant stage, for which no effective treatment is available. Recent approval and use of next generation anti-androgens such as enzalutamide and abiraterone has significantly expanded the therapeutic options available. But, even though they provide overall survival benefit to patients, clinical observations document resistance developing, which will curtail the duration of efficacy. Alternative splicing of the androgen receptor to generate hormone- insensitive variants is postulated to be one of the main mechanisms which contribute to development of resistance to enzalutamide. In this proposal, we hypothesize that microRNA circuits such as the Let- 7c:Lin28 axis modulate development of resistance to currently used anti-androgen therapeutics in PCa. In aim 1, we will analyze the mechanisms mediating the effects of the Let-7c:Lin28 axis on generation of AR splice variants and examine the clinical significance of expression of Let-7c:Lin28 in human PCa. In aim 2, we will characterize the effects of Let-7c:Lin28 axis on resistance to anti-androgens. In aim 3, we will determine the functional significance of hypermethylation of Let-7c promoter in prostate cancer therapy resistance. We expect to show that: 1) the Let-7c:Lin28 axis regulates sensitivity/resistance to anti-androgen therapies through regulation of the levels of hnRNPA1; 2) hypermethylation of Let-7c promoter suppresses Let- 7c expression in PCa, which can be reversed by inhibitors of DNA methyltransferases and 3) reversal of hypermethylation leading to restoration of Let-7c expression resensitizes PCa cells to anti-androgen therapeutics. These findings may establish the rationale for monitoring Let-7c:Lin28 status in patients being treated with anti-androgens.